VEGETATION TYPES. 487 



many other tropical plants are deciduous, casting their leaves 

 simultaneously, and in a few days new leaves are put forth. 

 There are no periodic extremes which require xerophytic struc- 

 tures for a season. Under these normal static conditions for 

 plant growth, we would expect a luxuriant development and a 

 vast overproduction with a tendency to migrate outward into 

 subtropic and temperate regions, during which modifications 

 are slowly brought about in the evolution of species adapted to 

 different conditions. Under these normal uniform conditions 

 the plant develops an extensive foliage surface adapted to trans- 

 piration, which is not likely to become excessive because of the 

 high degree of humidity of the air as well as the abundant pre- 

 cipitation. 



950. Xerophytic structures. The physical factors which de- 

 termine the conditions for xerophytic vegetation are of two kinds: 



1. Those which decrease the water supply to the plant, or make 

 it very limited, as slight precipitation, porous soil of a texture 

 which does not readily hold water, lack of subterranean supply, 

 or "ground water," strong winds or continual sunshine, which 

 dry out the soil, low temperature, thin soil covering on rock sub- 

 soil, thin mulching of vegetation, steepness of slope which ad- 

 mits of rapid removal of water. 



2. Those which accelerate the loss of water from the plant, as 

 dry air, high winds, high temperature if accompanied by dry- 

 ness of air, low atmospheric pressure, intense illumination, 

 absence of tall vegetation which affords protection to shade 

 plants. 



In the adaptation of plants to dry conditions modifications 

 have taken place along many different lines. These modifica- 

 tions are all designed for the same purpose, to conserve water 

 for the plant, to increase absorption by the roots, and to decrease 

 transpiration by the aerial parts. 



951. Lessened transpiration. This is brought about in thf 

 case of xerophytic plants in four general ways: 



i. Reduction of leaf surface. This is usually accompanied by 

 a thickening of the leaf, so that for the same mass of leaf sub- 



